<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    alert(d);//返回结果undefined
    var a=15;
    var b="hello";
    c=true;
    function fun(e) {
        a=20;
        alert("hello");
    }
    fun();
    var d=35;
</script>
</body>
</html>